Conventional shape memory alloy technology and heat treatment technology as well as the shape memory effect of materials were reported by F. Johnson in 1920. The shape memory effect of nickel-titanium alloy was reported by W. J. Buehler et al. (US Naval Ordnance Laboratory) in 1963. In 1966, J. A. Zijderveld was the first to use phase transformation theory to explain the shape memory effect. The shape memory effect can cause a material to switch between two shapes through temperature variation or provide a material with super-elasticity (elasticity that is much greater than that of a conventional material of the same type) through phase transformation. A generic nickel-titanium shape memory alloy is a binary alloy composed of nickel and titanium in equal atomic ratios. It is possible to adjust the characteristic phase transformation temperature and properties by appropriately adjusting the Ni—Ti ratio. In some cases, other elements, such as copper or niobium, replace part of the titanium to change its properties or phase transformation temperature.
In order to produce the shape memory effect, it is necessary to rigorously heat-treat the nickel-titanium alloy. Usually, a shape is first produced through a high-temperature heat treatment (hot rolling, hot forging, or hot extraction), followed by cold deformation to the required shaped. Then, said shape is fixed, and the temperature is kept in the range of 420-550° C. for a certain period of time. In this case, the part can switch between the aforementioned two shapes.
In 1991, Parodi and colleagues used a nickel-titanium alloy to treat abdominal aortic aneurysms. In 1993, Cragg and Dake used a Dacron-coated nickel-titanium alloy as the material for stents. Some of these designs exploit the shape memory effect, while others exploit super-elasticity.
It is well known that nickel is highly toxic to biological organisms. Although nickel-titanium alloy is not as toxic as nickel, research has shown that in many cases, nickel-titanium is harmful to the human body. Its long-term effects are still being studied. Titanium, on the other hand, is a metal that exhibits very good biocompatibility. Therefore, great efforts have been made to cover surfaces with titanium or titanium compounds and other materials that exhibit good biocompatibility. The methods that have been used include the following: 1) metal or metal ceramic is formed on a surface, for example, TiN is formed in a nitrogen-containing atmosphere or medium at high temperature or TiN is formed on the surface by the injection of nitrogen ions; 2) a polymeric material, such as Teflon, is coated on the surface; 3) an inorganic compound, for example, hydroxyapatite is sprayed on the surface. The surface treatment of shape memory alloys has the following difficulties:
a) temperature control: processing should not destroy the crystal structure of the material; otherwise, the shape memory effect or super-elasticity will be reduced or disappear;
b) toughness of the surface coating: since a shape memory alloy will undergo shape transformation during use, the surface coating must be able to deform with the deformation of the nickel-titanium alloy;
c) good adhesion: the surface coating must deform in response to the deformation of the nickel-titanium alloy without delaminating;
d) low transformation resistance: the surface coating should not hinder the shape transformation of the nickel-titanium;
e) low porosity: only a compact film can effectively improve corrosion resistance and biocompatibility and reduce or prevent elution of the nickel into the biological organism.
The objective of the present invention is to provide a method for coating TiN on the surface of medical devices made of nickel-titanium alloy. The obtained coating is compact, has high adhesion, and can deform with the deformation of the nickel-titanium alloy a finite number of times without cracking or delaminating. Its resistance to deformation of nickel-titanium is within an acceptable range. The treatment process does not adversely affect the shape memory effect or super-elasticity of the nickel-titanium alloy.